C++ program to calculate Cost of electricity bill with input of meter reading (logic explained)

Here is the C++ program to calculate Cost of electricity bill with input of meter reading.  It takes inputs of current and previous reading of meter and calculates the bill. You all should have in mind how it works exactly. Here is the algorithm.

Steps

User side:

1: User asked to enter the previous reading of meter

2: User asked to enter the current reading of meter 

3: User displayed the cost of the bill

Developer side

1: Declare variables that are going to be used in program.

2: Input the previous reading of meter from user.

3: Input the current reading of meter from user.

4: Calculate units. (the difference of current and previous readings)

5: If the units are less than 100 then calculate cost by multiplying units with 6 and there is no surcharge on it.
6: If the units are greater than one hundred  and less than three hundred then calculate cost by multiplying units with 7.5 and calculate surcharge using following formula surcharge=(10/100)*cost;. Total cost is cost plus surcharge  on it.
7: If the units are greater than three hundred   then calculate cost by multiplying units with 9 and calculate surcharge using following formula  surcharge=(20/100)*cost;. Total cost is cost plus surcharge  on it.
8: Display the cost.
9: Program ends here.

Code:

#include<iostream>

using namespace std;

void main()

{      //variable declaration

       int unit,preading,creading;

       float surcharge;

       double cost;

       //input of variables

       cout<<"Enter preveious reading"<<endl;

       cin>>preading;

       cout<<"Enter current reading"<<endl;

       cin>>creading;

       unit=creading-preading;

       cout<<"Units="<<unit<<"units"<<endl;

       if(unit<=100)

              {

                     cost=unit*6;

              }

       else if(unit>=101&&unit<=300)

                     {

                           cost=unit*7.5;

                           surcharge=(10/100)*cost;

                           cost=cost+surcharge;

                     }

              else if(unit>300)

                           {

                                  cost=unit*9;

                                  surcharge=(20/100)*cost;

                                  cost=cost+surcharge;

                           }

                     cout<<"cost of electricity bill for FAST-NU Faisalabad="<<endl;

                     cout<<cost<<endl;

       system("pause");

}

Output: